Helicobacter pylori was isolated in 1982 by B. Marshall and J. Warren using microareophilic conditions that had been developed to grow Campylobacter jejuni. H. pylori bacteria are S-shaped, gram negative bacilli 2-3.5 .mu.m in length and 0.5-1 .mu.m in width. The cell wall is smooth and adheres closely to the cytoplasmic membrane. H. pylori is responsible for the most common infection in the world. In developing countries 80% of the population is infected by the bacterium at the age of 20, while in developed countries H. pylori infection increases with age from &lt;20% in 30-year old people to &gt;50% in 60-year olds (Megraud, Gastroentrol. Clin. North Am., 1993, 22, 73-88; Taylor and Blaser, Epidemiol. REv., 1991, 13, Graham, J. Gastroenterol. Hepatol., 1991, 6, 105-113). The bacterium is probably transmitted by the oral-fecal route. Presently, there are two theories to explain the acquisition of H. pylori. One states that infection occurs during the first years of life and persists forever (cohort effect). The other purports that the acquisition of infectious bacteria occurs continuously, at a rate of 0.5-2% per year and, once established, the infection is chronic, possibly permanent. Risk factors for infection are crowding, poor hygiene and host-specific genetic factors.
Colonization of the mucosa of the stomach and the duodenum by Helicobacter pylori is today recognized as the major cause of acute and chronic gastroduodenal pathologies in humans (Blaser, Gastroenterology, 1987, 93, 371-383). The recognition of the infectious nature of the illness is having a major impact in the treatment of the disease that is shifting from the treatment of symptoms by anti-H2 blockers to the eradication of the bacterial infection by antibiotic regimen.
In spite of the unquestionable successes that will be achieved with antibiotic treatment, it should be remembered that this inevitably leads to the occurrence of resistant strains that in the long term will make antibiotics ineffective. This suggests that vaccination, which classically is the most effective way to prevent and control infectious diseases in a large population, could be used to prevent infection and possibly also to treat the disease (Rappuoli et al., J. Gastroenterol. Hepatol., 1993, 5 (Suppl. 2), 76-78).
Vaccine development requires understanding of a number of critical steps that are not yet fully studied. These are:
1) identification of the factors important for virulence, PA1 2) large-scale production and characterization of the virulence factors, PA1 3) development of appropriate animal models to test the virulence and immunogenicity of the molecules identified, PA1 4) development of an antigen delivery system capable to induce the type of immunity that is necessary to confer protection against infection and disease.
One of the most pressing needs is the development of an animal model in which to study H. pylori infection. Gnotobiotic piglets and beagle dogs can be artificially infected by H. pylori. In adult humans chronic H. pylori infection is associated with infiltration with neutrophils and mononuclear cells, whereas in infected children the lamina propria is infiltrated mainly by lymphocytes. Gnotobiotic piglets and dogs infected with H.pylori have a gastritis that is much more similar to the response seen in children with the inflammatory infiltration consisting mainly of lymphoplasmacytic cells with very few neutrophils. Thus these two infection models can only reproduce a pathological picture observed in a limited percentage of human patients. Non human primates have been reported to be naturally infected by H.pylori; the gastritis observed is more similar to that observed in the infected adult humans. Nonetheless, no evidence of H.pylori-dependent peptic ulceration has been reported in this animal model. A major problem with H.pylori infection of gnotobiotic pigs, dogs and monkeys is also the fact that these models are very expensive and have special housing requirements.
An animal model has been established by infecting germ-free mice with Helicobacter felis. The infection is characterized by an inflammatory response mostly consisting of neutrophil and eosinophil infiltration. However, H.felis lacks both cagA and vacA genes, is not adherent to epithelial cells and even a persistent infection of the mouse gastric mucosa does not give rise to ulcerations. Thus the H.felis model of infection in the mouse cannot give any direct information relating to H.pylori-specific pathogenic determinants. H.felis has also been reported to infect the gastric mucosa of dogs where it induces a cell infiltrate mainly consisting of mononuclear lymphocytes.
H. mustalae natural infection of North American ferrets causes chronic inflammation to the host. H. mustalae infection is often associated with ulcers but this bacterium does not synthesize a vacuolating cytotoxin. Moreover, the use of this model may not be easily available to most laboratories.
Gastrospirillum hominis, now re-named H. heilmanii, has been found associated with gastric mucosae of many domestic animals. It has also occasionally been found in human gastric biopsies associated with gastritis, and can successfully colonize the laboratory mouse stomach. Of course, reliable mouse models would be preferred by most researchers in the field.
Recently, Karita et al. have described successful gastric colonisation of athymic nude mice and germ-free euthymic mice by isolates of H.pylori derived from human gastric mucosa (Karita et al., Am J. Gastroenterol. 86, 1596-1603, 1991; Ibid 89, 208-213, 1994). From the model of Karita, however, it is apparent that both the immunological system and the normal gastric flora of the mouse stomach have far-reaching effects on infection by H.pylori. Indeed, in euthymic mice the level of infection observed was far lower than in athymic mice. Moreover, non-germ-free mice proved impossible to infect on a permanent basis. Since H.pylori injects humans with functional immunological systems and normal gastrointestinal flora, the important effects of both of these factors must be taken into account in the development of an animal model. A euthymic mouse with normal intestinal flora which can be successfully infected by H.pylori would therefore be the ideal model.